The present invention relates to an interference reduction circuit arrangement for use in an FM stereo receiver.
A well known technique for interference reduction in FM stereo receiver disign is disclosed and illustrated in U.S. Pat. No. 3,739,285, "CIRCUIT ARRANGEMENT FOR SUPPRESSING INTERFERENCES IN AN FM RADIO RECEIVER" assigned to U.S. Philip Corporation. According to this technique, a gate circuit is provided which is blocked upon the occurrence of impulsive interference signals and a resonant circuit with the pilot frequency is provided in parallel.
With such an arrangement, FM reception signals derived from the gate circuit are free of the impulsive interference signals. Should the pulse width of the impulsive interference signals be longer, the gate circuit will remain blocked longer, removing the information signals and the 19 KHz stereo pilot signals developed at that period. Even if the information signals are removed with the gate circuit to some extent, this will not present any adverse effect in listening to music. The absence of the 19 KHz pilot signals, however, is not desirable to FM reception. To this end the 19 KHz resonant circuit is provided between the gate circuit and a buffer amplifier to form a pseudo 19 KHz pilot signal. The FM reception signals are applied to a 38 KHz subcarrier generator and an FM multiplexer through the buffer amplifier and a conventional amplifier, while the output of the subcarrier generator is applied to the FM multiplexer.
Though the interference reduction circuit offers satisfactory performances in a sense to suppress impulsive interference signals, the true 19 KHz stereo pilot signals disappear together with the interference. If the pseudo 19 KHz signal from the resonant circuit is not completely in agreement with the 19 KHz pilot signal received at the antenna, then secondary interference signals will happen by the on and off operation of the gate circuit.
Accordingly, the 19 KHz signal from the resonant circuit needs extremely severe adjustment. The resonant circuit of good Q value is expensive and a Q value adjustment circuit is additionally required. The Q value of the resonant circuit will vary more or less from its initial value for long use of an FM receiver.